A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Conventionally, in a lithographic apparatus, the position of a table, for example a substrate table, is measured using a laser interferometer. In this system, gratings are placed on the edge of the table and laser beams shined at the gratings. The reflection is measured by a sensor positioned next to the laser light source. A difficulty with such measurement systems is that they can suffer from changes in refractive index of the medium (e.g. air) through which the beam travels. Additionally, their accuracy is not that great, though their long term stability is high.
Recently a new type of position measurement unit, called an encoder, has been developed. An encoder is described, for example, in U.S. Pat. No. 6,639,686 (hereby incorporated in its entirety by reference). A 2D grating target is positioned on the table or above the area at which the table is being used for processes which require accurate positional measurement of the table. Encoders comprising an emitter and a receiver are positioned at the other of the table and above the table. This has the benefit that the distance any radiation beams need to travel is reduced (so that any change in refractive of the medium through which the radiation beam passes has a smaller effect) as well as it being easier, should it be necessary, to condition the environment through which the radiation beams pass.
Both types of positional measurement system are disclosed as being used in parallel in, for example, US 2007/0288121 and US 2008/0094592 (both hereby incorporated in their entirety by reference). Using both systems allows accurate measurement using the encoder while the table is positioned under the projection system, for example, while still allowing positional measurement of the table over its entire movement range (which includes an area which is not under the projection system).
The above combination of table positioning devices can be used in any lithographic apparatus. The system may be particularly desirably used in immersion or EUV lithographic apparatus.
In immersion lithography, it has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. In an implementation, the liquid is distilled water, although another liquid can be used. However, another fluid may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
In an immersion apparatus, immersion fluid is handled by a fluid handling system, device structure or apparatus. In an implementation the fluid handling system may supply immersion fluid and therefore be a fluid supply system. In an embodiment the fluid handling system may at least partly confine immersion fluid and thereby be a fluid confinement system. In an implementation the fluid handling system may provide a barrier to immersion fluid and thereby be a barrier member, such as a fluid confinement structure. In an implementation the fluid handling system may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid so the fluid handling structure may be referred to as a seal member; such a seal member may be a fluid confinement structure. In an implementation, immersion liquid is used as the immersion fluid. In that case the fluid handling system may be a liquid handling system. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.